Crystalline polypropylene is excellent in rigidity, heat resistance and surface glossiness, but is poor in impact resistance.
Therefore, various polypropylene compositions improved in the impact resistance have been heretofore proposed. For example, polypropylene compositions containing crystalline polypropylene and modifiers such as polyethylene and rubber-like materials are known. Examples of the rubber-like materials conventionally used include a non-crystalline or low-crystalline ethylene/propylene random copolymer (EPR), polyisobutylene and polybutadiene.
In order to improve the impact resistance by adding such a rubber-like material as mentioned above, a large amount of the rubber-like material must be added to polypropylene. However, polypropylene compositions containing a large amount of the rubber-like material are lowered in the rigidity, the heat resistance and the surface hardness, though they are improved in the impact resistance.
On that account, polypropylene compositions further containing inorganic fillers in combination with the rubber-like materials have been proposed to improve rigidity, as described in Japanese Patent Laid-Open Publications No. 58459/1985, No. 60154/1985, No. 233048/1986, No. 36348/1986, No. 235350/1987, No. 122751/1988, No. 150343/1988, No. 149845/1989, No. 204947/1989, No. 271450/1989, etc.
However, the polypropylene compositions containing a large amount of the rubber-like material have a limit in the improvement in rigidity by the addition of the inorganic filler, and therefore they cannot be applied to such uses as require high rigidity.
As a replacement of such conventional polypropylene compositions, Japanese Patent Laid-Open Publication No. 156842/1988 proposes a polypropylene composition comprising polypropylene of high stereoregularity and a polystyrene.polyethylene/polybutylene.polystyrene block copolymer (SEBS), which shows good rigidity and impact resistance. However, it has been desired to develop polypropylene compositions much more improved in rigidity and impact resistance according to uses.